Dry grinding processes are used in many industries, in particular coal-fired power generation and cement manufacture. In a coal-fired power station coal is pulverised then pneumatically conveyed to the boiler, for cement manufacture the ground meal is pneumatically conveyed to a kiln. For both cases oversize particles in the streams do not undergo a complete reaction. Measurement of the particle size in the feed is the first step to control of the grinding process. To date this has been done by sub-sampling of the pneumatic transport stream.